Cellular systems have become the main stream of mobile communication systems such as those for mobile telephones. In cellular system, a wide area is covered by combining plural areas (cells) that are of a communicable range of a base station, where a mobile station maintains communication by switching the base station with which communication is established, according to the movement of the mobile station. For example, a service of a third generation mobile communication system is currently provided based on a code division multiple access (CDMA) system. On the other hand, a next generation mobile communication system enabling communication at a higher speed is under consideration (see, e.g., Japanese Laid-Open Patent Publication Nos. 2010-154399, 2010-157994, 2010-9385, and 2009-246875).
For example, in the 3rd generation partnership project (3GPP), a long term evolution (LTE) and an LTE-advanced that is an advanced version of the LTE are under consideration. For the LTE-advanced, carrier aggregation is under consideration in which plural carrier frequencies (component carriers: CCs) are used combining the component carriers with each other aiming at improving transmission rate.
For the LTE, a configuration is under consideration in which the carrier aggregation is executed by, for example, combining plural existing cells taking into consideration compatibility with a conventional mobile station apparatus and simplicity of the configuration. In such a configuration, each of combined component carriers independently forms a single cell. Therefore, in the carrier aggregation, plural cells are combined and operate as one cell. A radio resource control (RRC) to control communication between the mobile station and a cell is set for the mobile station.
Therefore, in the carrier aggregation, the RRC is set for any one of the combined plural cells and this cell is referred to as “primary cell”. The cells other than the primary cell among the plural cells are each referred to as “secondary cells”. In a case where the mobile station communicates using such aggregation, when the primary cell changes consequent to variation of the communication quality, etc., the primary cell and any one of the secondary cells are exchanged with each other. The procedure for this exchange includes, for example, delivery and reception of key information in a secrecy process and, for example, a procedure for handing over is used. For example, a cell whose communication quality is the highest is set to be the primary cell to stably control the radio communication.
However, according to the conventional techniques, when the primary cell changes consequent to, for example, variation of the communication quality in a configuration to execute the carrier aggregation by combining the plural existing cells, a complicated procedure for the handing over is executed. Therefore, an instantaneous interruption or a delay of the communication may occur and a problem arises in that the radio communication based on the carrier aggregation becomes unstable.